test: fix test-debug-signal-cluster.js flakyness

Do not assume any order and buffering/atomicity of output from child
processes' debugger agents.

Fixes: https://github.com/nodejs/node/issues/3796
PR-URL: https://github.com/nodejs/node/pull/8568


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>